The present invention relates to an imaging lens for forming an image of an object on an imaging element such as a CCD sensor and a CMOS sensor. In particular, the present invention relates to an imaging lens suitable for mounting in a relatively small camera such as a smartphone and a cellular phone, a digital camera, an infrared camera, a digital video camera, an onboard camera, a network camera, a TV conference camera, a fiberscope, and a capsule endoscope.
In these years, some vehicles are equipped with a plurality of cameras for improving convenience and/or security. For example, in case of a vehicle equipped with a backup camera, a driver can see the view behind the vehicle on a display upon backing up. Therefore, the driver can safely move the car backward without hitting any objects including the ones invisible by the driver from inside. Such camera for mounting in a vehicle, so-called “onboard camera”, is expected to be continuously in more demand.
Such onboard cameras are often mounted in relatively small spaces, such as a back door, a front grille, a side mirror, and interior space thereof. For this reason, an imaging lens for mounting in an onboard camera is required to have a compact size. Further, the imaging lens for an onboard camera is required to attain high resolution suitable for higher pixel count of an imaging element and a wide angle of view to achieve a wider range of an image. However, it is difficult to attain a wider angle of view, as well as satisfying the requirements of smaller size and higher resolution, while satisfactorily correcting aberrations. For example, when downsizing of an imaging lens is attempted, refractive power of each lens tends to be strong, so that it is difficult to satisfactorily correct aberrations. Upon actual designing of an imaging lens, it is a key to achieve those requirements in a balanced manner.
For example, as the conventional imaging lens having a wide angle of view, an imaging lens described in Patent Reference has been known.
Patent Reference: Japanese Patent Application Publication No. 2011-145665
The conventional imaging lens includes a first lens, a second lens, a third lens, and a fourth lens. The first lens is negative and has a shape of a meniscus shape, directing a convex surface thereof to an object side. The second lens is positive and has a shape of a meniscus shape, directing a concave surface thereof to the object side. The third lens is positive. The fourth lens is positive. According to the conventional imaging lens, the second lens is made of a material having an Abbe's number between 23 and 40. The third lens is made of a material having an Abbe's number between 50 and 85. In addition, according to the conventional imaging lens, a ratio (f/D) of a focal length f of a whole lens system and a distance D from an incident surface on an object side to an image-forming surface is restrained in a certain range. As a result, it is achievable to attain a wider angle and downsizing of the imaging lens as well as satisfactory correction of a chromatic aberration.
Demands for an imaging lens with a wide angle of view become more and more various each year. Especially in these years, there are higher demands for cameras that enable users to take images of objects even in the dark. Therefore, in case of an imaging lens for mounting in a camera like this, it is necessary to ensure the imaging lens to have satisfactory optical performance in the dark. In order to take an image of an object in the dark, for example, it is necessary to irradiate near-infrared light to the object from the camera and take an image of the reflected light. However, the wavelengths of near-infrared light beams are longer than those of visible light beams. For this reason, in a case of general wide-angle lenses, a focal position of near-infrared light significantly changes relative to that of visible light in the dark and it is difficult to form an image of the object on an imaging element. Therefore, in case of an imaging lens for mounting in a camera such as the above, it is necessary to have an image-forming performance at a near-infrared light range as well as at a visible light range.
According to the conventional imaging lens of Patent Reference, although the number of lenses that composes the conventional imaging lens is as small as four, the angle of view is wide and aberrations are relatively satisfactorily corrected. However, in case of conventional imaging lens having wide angles, such as the one described in Patent Reference, it is difficult to achieve satisfactorily image-forming performance over wide wavelength range from that of visible light to that of near-infrared light. As one of solutions for such problem, an optical element may be inserted and removed between an imaging lens and an imaging element so as to adjust a focal length. However, to do so, the imaging lens or the camera has to include a mechanism for inserting/removing the optical element, which is not preferred in view of downsizing of the imaging lens and/or the camera.
Here, such problem is not unique to the imaging lens for mounting in an onboard camera. In case of security cameras, it becomes a “must-have” function to be able to take an image by infrared radiation so as to monitor after sunset. In case of portable devices such as smartphones, night vision cameras which enable users to take images in the dark are sold as an option. In case of digital cameras and digital video cameras, there are already products available in the market, which are equipped with a night vision function. In addition, in case of network cameras, TV conference cameras, and cameras of fiberscopes and capsule endoscopes, there are products available, which have a function of taking images in a near-infrared range. The above-described problems are common among imaging lenses for mounting in relatively small-sized cameras.
In view of the above-described problems in the conventional techniques, an object of the present invention is to provide an imaging lens that can attain a wider angle of view and satisfactory correction of aberrations, while achieving a small size.
Further objects and advantages of the present invention will be apparent from the following description of the invention.